In steel making furnaces, including electric, basic oxygen process, or open hearth process, finished molten steel is normally transferred to a pouring ladle. Such ladles are generally supported by trunions or pivots and carried by an overhead crane to pour molten metal into individual ingot molds or into strand-casting equipment for continuous casting. For pouring molten steel from the ladle at a controlled rate, it has long been common practice to pour through a bottom nozzle, toward which the bottom of the ladle is normally tipped. Flow through the nozzle is controlled either by an internal stopper or an external slide gate. Due to maintenance and repair problems, particularly when the ladle is used as a holding vessel for continuous casting, external slide gate arrangements have now largely replaced the internal stopper system in most large ladles. Generally, the stopper requires replacement after each pour. Additionally, full shut-off of metal flow may be difficult to obtain where the stopper and actuating rod are immersed in molten metal during prolonged pours. Further, it is desireable not to cool the ladle significantly between pours. In a typical ladle slide gate, a movable nozzle in the gate portion is aligned with the stationary ladle nozzle in the ladle bottom for pouring through it.
While bottom slide gates significantly simplify nozzle and ladle construction, at the end of each pour the refractory gate plate, including the immoveable nozzle carried by it, must be inspected and usually replaced. This normally requires the vessel to be tipped and rested on its side for ready access. After inspection or replacement, the gate may be closed and the vessel returned to its erect position. In such righting of the ladle, portions of metal, refractory spallings, or slag remaining in the ladle may fall into the bottom stationary nozzle. Such material in the stationary nozzle frequently interferes with free flow of liquid metal when the ladle is loaded and the slide gate opened to pour the charge. Since such ladles hold on the order of 50 to 100 tons of molten steel, visual inspection of the nozzles after filling is not practical so that there is no way to assure full, quick flow of metal when the gate is opened. Further, after the vessel is returned to an erect position the stationary nozzle portion is normally filled with sand to protect the refractory materials of the stationary nozzle and the gate covering it while the vessel is being filled and moved to a pouring position. Where inadvertently the nozzle is partially plugged with pieces of steel, slag or the like, before sand is added or the sand is damp (or includes clay material), the sand body impedes full, quick flow, rather than aid in starting such casting flow, either for strand-casting (continuous casting) or into individual ingot molds. Such ladles are also known as metal holding vessels since they may be used for teeming molten steel into a continuous casting arrangement, such as an intermediate holding vessel, called a tundish.
Where flow does not begin, the nozzle must be "lanced" or poked to begin flow. While it is sometimes possible to open the vessel nozzle with an oxygen lance such as where a basic oxygen process lance is available, added oxygen may seriously deteriorate the finished steel or damage the walls of the ladle. At times, even with mechanical poking of the nozzle no opening can be obtained, resulting in complete loss of the melt at a cost of several thousand dollars. Further, in mold pouring equipment where a shroud is used between the pouring nozzle and the mold to prevent degradation of the steel by air exposure, mechanical access may not even be possible. Thus, any failure to flow is catastrophic to the entire vessel contents.